In 2006, as part of the Clean Air Act, the Environmental Protection Agency (EPA) passed legislation to limit the sulfur content of roadway diesel fuels to ≦15 ppm by 2010. This regulation was developed to diminish the negative impact of SOx emissions on the environment. SOx is responsible for acid rain, and is a principle component of industrial smog. The current commercial technology for removing sulfur compounds from petroleum fuels is hydrodesulfurization (HDS). HDS uses hydrogen in the presence of a catalyst (typically Co—Mo supported on alumina) to convert thiols, sulfides, disulfides and benzothiophenes to sulfur-free hydrocarbons and hydrogen sulfide. Operating temperatures typically range from 260 to 400° C. and 3-5 MPa hydrogen pressure. The sulfur-free hydrocarbons are recombined with petroleum streams, while the hydrogen sulfide is converted to elemental sulfur in a Claus plant.
HDS is very effective at desulfurizing lighter petroleum streams such as gasoline and naphtha, but sulfur-removal from heavier streams such as kerosene and gas oil present a greater challenge. The reason for this is that heavier streams (boiling range>150° C.) contain sulfur in the form of benzothiophenes (bp 200-350° C.). Benzothiophenes (BT's) and their alkylated derivatives are more difficult to hydro-treat than the sulfides, disulfides and thiols contained in the lighter streams. Therefore higher temperatures and hydrogen pressures are required to remove BT's from heavier petroleum streams using HDS technology.
HDS technology will continue to be challenged in the future as environmental laws become more stringent and petroleum supplies become more heavy and sour. The problem facing the oil industry is that meeting current and future sulfur regulations also forces the industry to increase its green house gas (CO2) emissions. This is based on the fact that using hydrogen gas to remove sulfur from oil is very energy intensive and leaves a large CO2 emissions footprint. Therefore an alternative desulfurization technology is needed to either supplement or replace HDS. The most promising candidate is oxidative desulfurization (ODS). ODS technology uses an oxidant, often in the presence of a transition metal catalyst, to convert sulfides to sulfones under relatively mild conditions (50-90° C., atmospheric pressure). The sulfones are then removed from the petroleum stream by distillation, extraction, adsorption, etc. Therefore ODS serves to greatly reduce CO2 emissions associated with the current HDS technology.
Most ODS attempts employed hydrogen peroxide (HP) as oxidant. Examples of catalytic systems that use HP are: H2O2-formic (or acetic) acid, titanium silicate, vanadium silicate, vanadium oxides supported on alumina, polyoxometalates, sodium tungstate and tungstophosphoric acid. However, HP is expensive and presents an economic barrier to ODS commercialization. An oxidant derived from air may be employed for ODS to be commercially viable.
Organohydroperoxides offer a good alternative to HP because they may be generated on site from air using existing petroleum streams, and their oxidation byproducts may be recombined with petroleum streams as oxygenates or sold in the fine chemicals market. To date, tert-butyl hydroperoxide (TBHP) has been the most frequently used alkyl hydroperoxide in ODS studies because it is a common industrial reagent.
Examples of catalytic systems that have shown successful use of TBHP in ODS studies include: Re(V) oxo complex/SiO2, MoO3/AlO3, and WOx/ZrO2. Cumyl hydroperoxide (CHP) has also been used in a study where ODS of model oil was performed in the presence of differing metal oxides (Fe, Co, Cu, V, Mo, Zr, Ti). Still other alkyl hydroperoxides used in ODS studies include cyclohexanone hydroperoxide (CYHPO) and tert-amyl hydroperoxide (TAHP).